Monitoring the battery state is the basic function of BMS. This includes the estimation and calculation of some parameters, including voltage, current, temperature, power, calculation of remaining capacity (SOC), reporting of battery deterioration (SOH), and SOP (state of power). Batteries power our electric vehicles, store renewable energy, and keep our mobile devices running. In this high-tech era, three acronyms—SOC, SOH, and SOP—hold the keys to ensuring these powerhouses operate efficiently and last as long as possible.
State of Charge (SOC), State of Health (SOH), and State of Power (SOP) are terms that might sound like technical jargon, but they are the parameters of battery states. Whether you’re an electric vehicle enthusiast, a renewable energy advocate, or simply someone curious about the technology that powers our world, grasping the distinctions and relationships between SOC, SOH, and SOP is paramount. This article will provide an in-depth analysis of the meaning of these three terms and their differences and importance in BMS.
PART 1: Battery State of Charge (SOC)
A. Definition of SOC
In BMS, the most important parameter is SOC (the state of charge). The remaining power we usually see when riding an electric bike or using a cell phone is the result of the BMS’s calculation of SOC. It can be commonly understood as how much power is left in the battery, whose value range is between 0-100%. SOC=0 means the battery is fully discharged. And SOC=1 means the battery is fully charged.
B. How to measure SOC
Measuring the SOC in the battery is a complex task that depends on the battery type and application. Accurately determining SOC helps improve system performance and battery lifetime by avoiding overcharging or over-discharging. However, precisely estimating SOC in a battery management system is challenging because the chemical and physical processes during charging and discharging are intricate.
The general approach to measure SOC is to closely monitor the current flow in and out of the battery and the voltage of each cell. This data is combined with battery specifications like temperature, age, and manufacturer charging/discharging characterizations. With an initial SOC estimate, the system tracks coulombs in and out to provide an updated SOC. However, factors like self-discharge and current leakage can reduce accuracy over time.
There are 3 methods used for SOC measurement: the Coulomb counting method, the voltage method, and the Kalman filter method. These methods are applicable to all battery systems, especially HEVs, EVs, and PVs.
- Coulomb Counting Method
The most prevalent approach for determining battery state of charge (SOC) is coulomb counting, also known as ampere-hour counting or current integration. This method uses the following soc battery formula:
In the formula, SOC(t0) represents the initial state of charge, Crated is the rated capacity of the battery, Ib signifies the battery current, and Iloss denotes the current consumed by loss reactions.
The Coulomb counting method is employed to determine the remaining battery capacity by simply tracking the charge that enters or exits the battery. The accuracy of this approach primarily hinges on the precise measurement of battery current and an accurate estimate of the initial SOC. When a known battery capacity is available, which may be either memorized or initially approximated based on operating conditions, the SOC of the battery can be computed by integrating the charging and discharging currents over the operational periods.
It is important to note that the actual charge that can be released during charging and discharging is always less than the stored charge due to losses incurred in these processes. In simpler terms, there are inefficiencies during both charging and discharging and these, in conjunction with self-discharge, contribute to accumulating errors. These factors must be considered in order to be more accurately estimate SOC. Furthermore, it is advisable to regularly recalibrate SOC and consider the diminishing releasable capacity to enhance precision in estimation.
- Voltage Method
The State of Charge (SOC) of a battery can be ascertained through a controlled discharge test. The voltage-based method relies on translating the battery voltage reading into an equivalent SOC value using the established discharge curve (voltage vs. SOC) specific to the battery in question. Nevertheless, it’s important to note that the voltage is significantly influenced by the battery’s electrochemical kinetics and temperature, primarily driven by the battery current. To enhance the precision of this method, one can apply a correction term linked to the battery current and refer to a lookup table detailing the battery’s open-circuit voltage (OCV) concerning temperature.
It’s worth mentioning that the voltage method encounters challenges related to maintaining a stable voltage range for the batteries, making its implementation complex. Additionally, the discharge test typically involves a subsequent recharge, rendering it time-consuming and less feasible for many applications. Another drawback is that during testing, the system’s functionality is interrupted (an offline method), in contrast to coulomb counting, which operates in real-time (an online method).
- Kalman Filter Method
The Kalman filter is a versatile algorithm designed to estimate the internal states of dynamic systems, and it can be effectively utilized for the estimation of State of Charge (SOC) in batteries. This involves incorporating the desired unknown variables, such as SOC, into the system’s state description. The Kalman filter then deduces these values and provides error margins for the estimations. As a result, it evolves into a model-based state estimation technique with an error correction mechanism, facilitating real-time SOC predictions.
To enhance its capacity for real-time State of Health (SOH) estimation, an extended Kalman filter can be employed. Notably, the extended Kalman filter is used when the battery system exhibits nonlinear behavior and necessitates a linearization step.
It’s important to recognize that while Kalman filtering is an online and dynamic method, it demands a suitable model for the battery system and precise parameter identification. Additionally, it requires substantial computational resources and an accurate initialization process.
PART 2: Battery State of Health (SOH)
A. SOH Battery Meaning
SOH is a measure of how well a battery performs compared to its original specifications when it was brand new. It provides insights into the aging process of a battery and its ability to continue operating effectively. The BMS SOH is expressed as a percentage, where 100% represents a battery in perfect health, and values below 100% indicate a decline in performance over time. Just as we assess our well-being through regular check-ups and physical examinations, batteries also require ongoing evaluation to determine their State of Health (SOH).
B. Why SOH Matters
Longevity: A battery with a high SOH will have a longer operational life. This is particularly important for applications such as electric vehicles and renewable energy storage systems where batteries last for years.
Safety: As batteries age, their internal components can degrade, potentially leading to safety concerns. Monitoring SOH allows for early detection of potential issues and preventive measures to ensure safe operation.
Cost-Efficiency: Replacing batteries prematurely can be costly. Monitoring SOH helps optimize battery replacement schedules, reducing the overall cost of ownership.
C. What Factors Will Influence Battery State SOH
Cycle Count: The number of charge and discharge cycles a battery undergoes can degrade its SOH. Each cycle causes a small amount of wear and tear, eventually leading to reduced capacity.
Depth of Discharge (DoD): Deeper discharges, where a battery is drained to lower states of charge, can accelerate SOH degradation.
Temperature: Operating a battery in extreme temperatures, whether hot or cold, can harm its health. Extreme temperatures affect the chemical reactions inside batteries, causing performance issues. High heat speeds up the reactions but leads to faster battery degradation over time. Low temperatures slow the reactions and reduce battery performance.
Overcharging or Over-discharging: Both scenarios can be detrimental to a battery’s SOH. Overcharging can cause batteries to overheat and lose capacity. Letting batteries discharge too deeply can permanently damage them.
D. How to Measure SOH
- Calculate SOH from a table based on the number of battery cycles
This method requires testing the table corresponding to the number of battery cycles and capacity retention in the laboratory in advance. The current cycle times are obtained by dividing the accumulated discharge capacity by the rated capacity (C0). Then look up the table of the correspondence between the number of cycles and the capacity retention rate to get the current capacity retention rate, which is the SOH value.
- Calculate SOH based on charging capacity
When the vehicle is charging, record the charging start SOC (SOC0) and charging end SOC (SOC1). Then calculate the capacity of the charged battery (C1) by the ampere-time integration method, and calculate the SOH value by the following formula.
The charging start SOC and end SOC have a large impact on the accuracy of the calculation, and the data with lower start SOC and full charge are generally selected for calculation to improve the accuracy.
PART 3: Battery State of Power (SOP)
A. Defining Battery State of Power (SOP)
SOP means measuring the power output capability of a battery at any given time. It is expressed in terms of a percentage, with 100% representing a fully capable battery, ready to deliver its maximum power output. Lower SOP percentages indicate that the battery’s power delivery capacity is reduced.
B. The Importance of SOP
Performance: In applications like electric vehicles, batteries must deliver power rapidly to accelerate, climb hills, or execute emergency maneuvers. Knowing the SOP helps ensure that the battery can meet these performance demands.
Safety: In emergency situations, where a sudden burst of power is required, a battery with a low SOP may not deliver adequately, potentially compromising safety. SOP monitoring helps mitigate these risks.
Efficiency: SOP monitoring enables efficient power management. For example, in renewable energy systems, a battery with a high SOP can discharge quickly during peak demand periods, improving the efficiency of energy distribution.
C. What Factors Will Influence SOP
Temperature: Extreme temperatures can affect a battery’s power output. High temperatures can improve power delivery temporarily, but it may cause long-term degradation. Conversely, cold temperatures can reduce power output.
Internal Resistance: As batteries age, their internal resistance can increase, leading to reduced power output. This can be accelerated by factors such as high cycling rates and overcharging.
Capacity Loss: A battery with reduced capacity will also have a lower SOP because it cannot sustain high power delivery for as long as a fully charged battery.
PART 4: Key Differences and Relationships
Now that we’ve explored the individual concepts of State of Charge (SOC), State of Health (SOH), and State of Power (SOP), it’s time to delve into how these parameters are both distinct and interconnected. Understanding the key differences and interrelationships between these three elements is essential for effective battery management and optimization.
A. Key Differences between Battery State SOC, SOH, and SOP
State of Charge (SOC): SOC primarily measures the remaining energy capacity of a battery. It provides information about how much energy is left, expressed as a percentage of the battery’s total capacity. SOC tells us whether the battery is full or partially depleted.
State of Health (SOH): SOH, on the other hand, assesses the overall condition and aging of the battery. It informs us about how well the battery is performing compared to its original specifications when new. SOH is expressed as a percentage and reflects the battery’s wear and tear over time.
State of Power (SOP): SOP focuses on the battery’s ability to deliver power effectively at a given moment. It is also expressed as a percentage and tells us how capable the battery is of providing the required power output. SOP varies depending on factors like temperature, internal resistance, and capacity.
B. Interrelationships and Impact
SOH and SOC: The SOC of a battery can affect its SOH. Repeated deep discharges or overcharges, often reflected in low SOC levels, can accelerate the aging process and reduce the battery’s overall health. Maintaining a battery within a safe SOC range can extend its SOH.
SOC and SOP: The SOC directly influences the battery’s ability to deliver power. A battery at a higher SOC typically has more power available for immediate use. However, maintaining high SOC for prolonged periods can be detrimental to SOH, so a balance must be struck.
SOH and SOP: As a battery’s SOH degrades over time, its ability to deliver power can diminish. The internal resistance of the battery may increase as it ages, impacting SOP. For example, an old battery with a low SOH may struggle to provide power as effectively as a new one.
Leveraging cutting-edge algorithms and software, MOKOEnergy BMS enables precise control and monitoring of battery packs for unparalleled performance, safety, and longevity. Our flexible BMS architecture can be tailored to diverse applications, spanning electric vehicles, energy storage systems, and beyond. Strict quality assurance and exclusively high-grade components from trusted suppliers underscore our commitment to excellence. With a focus on continuous R&D and innovation, MOKOEnergy aims to be at the leading edge in terms of BMS technology advancements. So talk to our expert and we’ll let you know how to choose your BMS solutions.